Printing plates such as flexographic (flexo) plates coated with a photopolymer resin layer are typically digitally imaged or patterned using a modulated and rasterized laser beam or beams, or an array of light produced by an illuminated and re-imaged spatial light modulator array, in a machine such as a computer-to-plate (CTP) exposure system. Most flexo plates are not manufactured in an oxygen-free environment and are not overcoated with an oxygen barrier layer such as polyvinyl alcohol. In addition, oxygen-contaminated components are distributed throughout the photopolymer. In this case, there is an abundance of dissolved oxygen throughout the photosensitive resin that inhibits photopolymerization. And, if oxygen is removed from such a flexo plate, ambient oxygen will diffuse back into the resin.
The third dimension of the exposed and developed flexo plate, especially the sidewall slope of a patterned feature, is a critical determinant of exposed image quality. For an isolated dot on such a plate, a wide base and a narrow flat top are preferred. This is achieved with conventionally exposed ultraviolet sensitive photopolymer resins by first “bumping” the plate, i.e., uniformly flooding the entire plate with ultraviolet light to photometrically consume the dissolved oxygen throughout the resin, followed by exposing with patterned light to polymerize selected features on the plate. The exposed plate is chemically developed, leaving a residual image of polymerized resin attached to the plate substrate. The bumping process initiates a chemical reaction in the unexposed plate with a time constant that in thick flexographic resins has been measured in several seconds. Immediately after the bump is applied, ambient oxygen begins to diffuse back into the plate from the surface. This leads to a higher oxygen concentration in the resin closer to the surface (top of plate) than to the substrate (bottom of plate) so that photopolymerization inhibition is greater near the surface. Thus, when the patterning exposure is performed, the dot base is more polymerized than the top resulting in a wider base and a narrower top.
For a successful flexo CTP system, it is necessary to tailor the oxygen concentration throughout the resin cross section with a pre-exposure system that adjusts the bump irradiance and the elapsed time from the bump until the patterning exposure.
It is well known to pre-expose the whole printing plate using a separate bumping station comprising an ultraviolet or near-ultraviolet source and then to transfer the printing plate to a conventional film-based exposure system or a maskless CTP exposure system for patterning. This method works well for a flood bump exposure combined with a flood pattern exposure but fails when the bump simultaneously exposes the full plate area followed by a pattern exposure of the plate one section at a time. Thus, the best exposure performance and exposure latitude occurs when the delay between the bump and the patterning is nearly constant at all locations on the flexo plate.
U.S. Pat. No. 5,455,416, incorporated herein by this reference, discloses a preexposure device wherein a linear LED arrangement preexposes a printing plate traveling on a conveyer under the device. When and how the printing plate is then subjected to full exposure, however, is not disclosed in the '416 Patent.
In U.S. Pat. No. 6,262,825, also incorporated herein by this reference, an apparatus is disclosed in which a beam of laser radiation is split to provide the patterning exposing beam which trails the “pre-exposing” beam to limit the time period between the bumping and the pattern exposure process. In some respects, the “pre-exposing” laser beam can be thought of as wasted laser power due to the requirement of a more powerful laser than is required only for pattern exposure.
Furthermore, U.S. Pat. No. 6,262,825 teaches only a single backscan (bump) beam in advance of the imaging (patterning) beam. In the case of flexographic platemaking, a minimum delay is required to permit the process of photometric consumption of oxygen to reach chemical equilibrium. This defines a minimum temporal separation between the backscan and the imaging beam. However, too large a temporal separation results in an excessive amount of oxygen that diffuses back into the top layer of the resin. For high-speed plate imaging, a single backscan beam does not easily satisfy these two boundary conditions and can result in a small or non-existent exposure process window. However, two backscan beams or two bands of illumination, one that delivers a large dose to consume the oxygen dissolved throughout the flexo resin followed many seconds later by the other beam or band of illumination that delivers a smaller dose to consume the oxygen that has re-dissolved into the flexo resin top surface, have the potential to better tailor the oxygen concentration throughout the resin cross section.